1. Field of the Invention
The invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display device which is capable of reducing power consumption.
2. Description of the Related Art
In order to overcome various shortcomings of the cathode ray tube (CRT), various types of flat panel display devices having a reduced weight and size compared to that of a CRT have been developed. These flat panel display devices include liquid crystal display (LCD) devices, field emission display (FED) devices, plasma display panels (PDP), and organic electroluminescent (EL) display device.
The structure and manufacturing process associated with a PDP are relatively simple, and so a PDP is often used when a large size display surface is required. However, the light emitting efficiency and brightness of a PDP tend to be low and an enormous amount of power is consumed by the PDP.
LCD devices are mainly used with laptop computers, and demand for LCD devices has increased. However, it is difficult to use an LCD device for a large display surface, and power consumption is high due to backlight requirements. Additionally, light loss in an LCD device is high due to optical members such as a polarized light filter, a prism sheet, a diffusion sheet and the like which may be included in the LCD device, and the viewing angle associated with an LCD device is narrow.
Electroluminescent devices (hereinafter, referred to as “EL devices”) can be either an organic EL device or an inorganic EL device based on the material which forms a light emitting layer of the device. As the EL device itself emits light, response time is short, light emitting efficiency and brightness are high, and viewing angle is wide when compared to other flat panel display devices. However, an inorganic EL device has a higher level of power consumption when compared to an organic EL device, high levels brightness cannot be obtained by the inorganic EL device, and various levels of R (red), G (green) and B (blue) light cannot be emitted. On the other hand, the organic EL device may be driven by a variety of levels of low direct current, has a rapid response time, and can obtain high brightness and emit various levels of R, G and B light, and so is well suited for the next generation of flat panel display devices.
FIG. 1 is a schematic view of a conventional organic EL display device.
The conventional organic EL display device includes an organic EL display panel 20 with anode columns DL1-DLm, cathodes SL1-SLn perpendicular to the anode columns DL1-DLm, and first and second organic EL diodes 10a and 10b formed at common sections of the anode columns DL1-DLm and the cathodes SL1-SLn. A non display area with a data pad 24 is connected to the anode columns DL1-DLm via data lines (not shown), and a scan pad 22 is connected to the cathodes SL1-SLn via scan lines (not shown). The data pad 24 and the scan pad 22 are connected to a tape carrier package (not shown) in which a data driving section (not shown) for generating a data signal and a scan driving section (not shown) for generating a scan signal are provided. A data signal transmitted from the data driving section is supplied to the anode columns DL1-DLm via the data pad 24 and the data lines, and a scan signal transmitted from the scan driving section is supplied to the cathodes SL1-SLn via the scan pad 22 and the scan lines.
Each anode column (for example, DL1) comprises first and second anodes DL1-1 and DL1-2 adjacent to each other, and each cathode (for example, SL1) is divided into first and second sub cathodes SL1-1 and SL1-2 spaced apart from each other as shown in FIG. 1. Accordingly, a scan signal supplied from the scan driving section to one of the cathodes (for example, SL1) is supplied simultaneously to the first and second sub cathodes SL1-1 and SL1-2.
In a first organic EL diode 10a, an anode electrode is connected to the first anode DL1-1 of the anode column DL1, and a cathode electrode is connected to the first sub cathode SL1-1 of the cathode SL1. In the second organic electroluminescent diode 10b, an anode electrode is connected to the second anode DL1-2 of the anode column DL1, and a cathode electrode is connected to the second sub cathode SL1-2 of the cathode SL1. Therefore, though the scan signal is supplied to the cathode SL1 from the scan driving section, the data driving section can drive the first and second organic EL diodes 10a and 10b independently. This operation is performed in all anode columns DL1-DLm and cathodes SL1-SLn.
When a negative scan signal is supplied to the cathodes SL1-SLn to which the cathode electrodes of the first and second organic electroluminescent diodes 10a and 10b are connected, and a positive data signal is supplied to the first anode (for example, DL1-1) of each anode column (for example, DL1) to which the anode electrode of the first organic EL diode 10a is connected and the second anode DL1-2 of each anode column to which the anode electrode of the second organic EL diode 10b is connected, the current flows based on a forward bias, and so the first and second organic EL diodes 10a and 10b emit light.
Each of the first and second organic EL diodes 10a and 10b is formed with a first EL cell R having red fluorescent material, a second EL cell G having green fluorescent material, and a third EL cell B having blue fluorescent material. Each organic EL diode corresponding to one pixel of the organic EL display device emits a color image for that pixel by combining the first EL cell R, the second EL cell G and the third EL cell B.
FIG. 2 is a detailed view of section “A” of FIG. 1. Simply for ease of discussion and illustration, only three (3) anode columns and one (1) cathode are shown in FIG. 2. As shown in FIG. 2, the cathode SL1 is positioned substantially perpendicular to the anode columns DL1, DL2, DL3, with a first EL cell R, a second cell G, and a third EL cell B formed at areas of the anode columns DL1, DL2, DL3 which correspond to the cathode SL1. Primary walls 8a and a secondary wall 8b are positioned parallel to the cathode SL1.
Each anode column (for example, DL1) adjacent first and second anodes DL1-1 and DL1-2, and a section of the cathode SL1 (that is, a section corresponding to the emitting area) is divided into first and second sub cathodes SL1-1 and SL1-2 separated from each other by the secondary wall 8b. Accordingly, two emitting light areas are formed at each area where an anode column (for example, DL1) and the cathode SL1 meet.
The primary walls 8a extend to a non-display, or non-emitting area, but the secondary wall 8b is formed only in the EL cell array area, or active region. After forming the walls 8a and 8b, organic emitting material is deposited through a mask on the EL cell array area of the substrate on which the walls 8a and 8b are formed, and so the first EL cell R, the second cell G and the third EL cell B are formed. Thereafter, the cathode SL1 is formed by depositing conductive material on an entire structure.
When a negative scan signal is supplied to the cathode SL1, i.e., the first and second sub cathodes SL1-1 and SL1-2, and a positive data signal is supplied to one anode (for example, DL1-1) of the anode column DL1 at the same time, the current flows to the cathode SL1 via the current path shown in FIG. 3 so as to emit light through a corresponding cell. In this type of organic EL display device, if the data current for driving the pixels in full white is applied to the first and second sub cathodes SL1 and SL1-2 through the anodes or if a relatively large data current is applied, the first and second sub cathodes SL1 and SL1-2 may not be able to withstand the load resulting from the supplied the current.
Since the scan signal is supplied to two sub cathodes SL1-1 and SL1-2 through the cathode SL1 formed at the non-emitting area, the number of cathodes formed may be half the number of sub cathodes so that each cathode has a larger width in the non-emitting area. This will decrease line resistance of each cathode SL1-SLn in the non-emitting area, thus decreasing power accordingly.
However, though the line resistance of the cathodes SL1-SLn is decreased, the line resistance of the first and second sub cathodes SLn-1-SLn-2 formed on the EL cell array area is not decreased, and so this method is limited in its ability to decrease power consumption of the organic EL device.